1. Field of the Invention
The present invention relates to DC brushless motors for high speed rotation with high efficiency.
2. Description of the Prior Art
FIG. 14 shows a conventional DC brushless motor 1 including a motor case or stator 2 which has a cylindrical side wall 3 and a pair of end walls; i.e., a front wall 4 and a rear wall 5, for closing the ends of the side wall 3. A plurality of exciting coils 6 are fixedly arranged on the inside of the side wall 3 in the form of a cylinder. A rotary shaft 8 extends outwardly from opposite ends of a rotor 7 in concentric relation to the rotor 7 and is supported for rotation by a pair of bearings 10 and 12 which are fitted in holes 9 and 11 of the rear wall 5 and the front wall 4, respectively. An annular support member 13 is provided within the motor case 2 to support a plurality of magnetic pole sensors 14 in the vicinity of the circumference of the rotor 7.
FIG. 15 shows the conventional rotor 7 including a cylindrical yoke 15 into which the rotary shaft 8 is put as a unit. A pair of arcked permanent magnets 16a with the N pole on the outside and the S pole on the inside and a pair of arcked permanent magnets 16b with the N pole on the inside and the S pole on the outside are bonded to the cylindrical yoke 15 so that their poles are arranged alternately.
In operation, the magnetic pole sensors 14 of the DC brushless motor 1 detect the magnetic pole position of the rotor 7, and a control circuit (not shown) conducts electric current through the corresponding exciting coil 6 to rotate the rotor 7 by the interaction between the electric current and the magnetic flux. The magnetic pole sensors 14 detect again the magnetic pole position of the rotated rotor 7, and the control circuit supplies electric current to a different exciting coil 6 to rotate again the rotor 7. The above operation is repeated to continuously rotate the rotor 7, the rotary power of which is taken out via the rotary shaft 8 of the motor 1.
As FIG. 16 shows, a protective member 17 made of a non-magnetic metal is provided to cover the arcked permanent magnets 16a and 16b for preventing the permanent magnets 16a and 16b from being pulled off from the rotor 7 by a large centrifugal force resulting from high speed rotation of the motor 1.
However, the maximum energy product and residual magnetic flux density of DC brushless motors using ferrite magnets are as small as 3.3 MGOe and 3.8 KG, respectively, so that it is necessary to raise the permeance of the magnetic circuit in order to produce satisfactory torque to drive the motor. For this reason, a large amount of magnet is required, making the motor size very large.
When the stress caused by the centrifugal force resulting from high speed rotation becomes greater than the material and bonding strengths of the permanent magnets of a scroll type compressor, the permanent magnets can be broken up and scattered.
When a protective member is used to cover the rotor in order to prevent the permanent magnets from being scattered, not only the manufacturing process is complicated but also the space between the rotor and the stator is increased by that much. Consequently, the magnetic resistance increases while the magnetic density decreases, resulting in the reduced efficiency.